The present invention generally relates to fabrication of semiconductor devices and more particularly to a method for growing a mixed crystal layer of a compound semiconductor material.
In the effort of developing high performance semiconductor devices, one encounters a persistent demand for a high quality semiconductor crystal. Particularly, there is a keen demand for a high quality crystal of compound semiconductor materials. The compound semiconductor materials generally have a band structure that provides a high electron mobility, and many of them show a direct transition of carriers that facilitates the absorption and emission of light. Thus, compound semiconductor devices are used for high performance semiconductor devices such as optical semiconductor devices including laser diodes and light emitting diodes, high speed semiconductor devices including HEMTs (high-electron-mobility transistors) or RHETs (resonant-tunneling hot-electron transistors) as well as quantum semiconductor devices that confine the carriers in a limited region such that a quantum mechanical effect of carriers becomes appreciable. As the compound semiconductor materials generally form a mixed crystal over a wide compositional range, one can construct various quantum structures that facilitates the quantum mechanical effect.
Generally, a compound semiconductor crystal is grown on a substrate crystal in the form of an epitaxial layer. In order to grow such an epitaxial layer, various processes are known, wherein so-called MBE (molecular beam epitaxy) and MOCVD (metal-organic chemical vapor deposition) processes are used extensively. In any of these processes, atoms of the elements that constitute the compound semiconductor material are supplied simultaneously to the surface of a substrate for deposition. For example, atoms of the group III elements such as Al or Ga and atoms of the group V elements such as As are supplied simultaneously to the surface of the substrate, wherein the atoms thus supplied are incorporated into respective crystallographic sites of the crystal layer on the substrate. In such a process, it is generally impossible to eliminate defects wherein a group III element such as Ga is incorporated into the site of a group V element such as As. While there are proposals to utilize such defects positively, various efforts are being made to eliminate the formation of such defects by depositing a layer of Ga atoms and a layer of As atoms alternately, one layer by one layer.
The atomic layer epitaxy (ALE) is a process that realizes such a monoatomic layer growth. The process relies upon a self-limiting effect that appears when a gaseous source material is supplied on an epitaxial layer, wherein the crystal growth stops substantially upon a growth of a monoatomic layer. For example, a source gas containing Ga is supplied on a substrate for depositing Ga thereon. Upon growth of a Ga monoatomic layer, the supply of the Ga source gas is interrupted and the reaction vessel in which the growth has been made is flushed. Further, another source gas containing As is supplied and a monoatomic layer of As is grown on the monoatomic layer of Ga grown previously. By repeating the foregoing steps, one can obtain a crystal layer substantially free from defects. When a GaAs layer is grown, trimethylgallium [(CH.sub.3).sub.3 Ga; TMG] is used for the source of Ga and arsine (AsH.sub.3) is used for the source of As. Upon supplying, the source gases cause a pyrolytic decomposition at the surface of the substrate and atoms of Ga and As are released as a result. Because of the foregoing self-limiting effect, the Ga and As atoms thus released form only a monoatomic layer in each of the steps. Such an art of atomic layer epitaxy is described in the U.S. Pat. No. 4,058,430, which is incorporated herein as reference. A similar atomic layer epitaxy is reported also for material systems other than GaAs such as InAs. When growing an epitaxial layer of InAs by the atomic layer epitaxy, trimethylindium [(CH.sub.3).sub.3 In; TMI] is used as the source of In.
In the atomic layer epitaxy, it will be easily understood that there exists an optimum condition, particularly an optimum temperature, for realizing the foregoing self-limiting effect. When the optimum temperature range has been exceeded, the desired monoatomic layer growth no longer holds. As the optimum temperature range for the atomic layer epitaxy changes depending upon the material system used for growing the crystal, there has been a difficulty for growing a layer of mixed crystal by the atomic layer epitaxy. For example, the optimum temperature for growing a GaAs layer by the atomic layer epitaxy of TMG and arsine is about 500.degree. C. On the other hand, the optimum temperature for growing an InAs layer by the atomic layer epitaxy of TMI and arsine is about 350.degree. C. In such a case, the monoatomic layer growth of In does not occur when the growth of the mixed crystal is made at a temperature of 500.degree. C. Instead of the monoatomic layer growth, a thick layer of InAs is formed by a single growth step. When the temperature of growth is set to 350.degree. C., on the other hand, the growth rate of the GaAs layer decreases substantially, resulting no substantial growth of the crystal layer. Because of such a difference in the optimum temperature of crystal growth, it has been difficult to provide the growth of mixed crystals according to the conventional atomic layer epitaxy. While it is possible to achieve the growth of a GaAs layer at a temperature of 350.degree. C. when triethylgallium [(C.sub.2 H.sub.5).sub.3 Ga; TEG] is used, this material does not show a clear self-limiting effect and the desired monoatomic layer growth is not obtained even when combined with the atomic layer epitaxy process that uses TMI and arsine.
Meanwhile, it has been known that the appearance of an ordered structure of In and Ga in an InGaAs layer results in a decrease of alloy scattering of carriers and a corresponding increase of the carrier mobility (Nakata et al., J. Crystal Growth 115, 1991, pp. 504-508). By using such an ordered InGaAs layer in combination with an n-type InAlAs layer, it is expected that an electron mobility that reaches as much as 161,000 cm.sup.2 /V.s is obtained for the two-dimensional electron gas formed at the heterojunction interface of the InGaAs layer and the InAlAs layer. Usually, such an ordered structure appears as a result of the thermodynamic equilibrium in the crystal and controlled by the temperature at which the crystal growth is made. Thus, a complete ordering of In and Ga is achieved only when the crystal growth is made at an extremely low temperature. However, the crystal growth at such an extremely low temperature is of course impossible.
By using the atomic layer epitaxy, one can obtain a completely ordered crystal artificially and without relying upon the thermodynamic equilibrium, by growing the crystal one atomic layer by one atomic layer. Such an artificially ordered structure forms an artificial superlattice structure. As the alloy scattering of the carriers is entirely eliminated in such an artificially ordered structure, it is expected that such a structure provides an electron mobility that reaches the order of 10.sup.6 cm.sup.2 /V.s. As already noted, however, because of the difficulty in growing mixed crystals by the atomic layer epitaxy, the growth of crystal layers having such an artificial superlattice structure has been unsuccessful.